Gene therapy, or genetic engineering, is the insertion, alteration, or removal of genes within an individual's cells and biological tissues to treat disease. It is a technique for correcting defective genes that are responsible for disease development. The most common form of genetic engineering involves the insertion of a functional gene at an unspecified location in the host genome. This is accomplished by isolating and copying the gene of interest, generating a construct containing all the genetic elements for correct expression, and then inserting this construct into a random location in the host organism.
Other forms of genetic engineering include gene targeting and knocking out specific genes via engineered nucleases such as zinc finger nucleases, engineered I-CreI homing endonucleases, or nucleases generated from TAL effectors. An example of gene-knockout mediated gene therapy is the knockout of the human CCR5 gene in T-cells in order to control HIV infection. The use of gene therapy for the treatment of HIV-1 infection received a huge boost when it was demonstrated that HIV-1 infection could be eradicated from an AIDS/leukemia patient who received an allogeneic hematopoietic stem cell transplant from a donor who had a homozygous deletion in the chemokine receptor gene CCR5 [2]. It is known that CCR5 is a primary co-receptor for HIV-1 entry, so the transplant patient was, in effect, given a protective therapy. Although this is a single patient result, the idea that repopulation of the hematopoietic system with cells resistant to HIV-1 infection can have a major impact on the disease is being investigated. Finding CCR5−/− compatible donors is a major challenge and as such this cannot be considered to be a treatment that will have broad applications. In addition, virus that mutates to CXCR4 tropism can infect the CCR5−/− cells and reactivate the infection.
Treatment of disease using genetic engineering has been met with limited success, as there are several challenges that prevent gene therapy from being more successful. Some of these challenges include (1) problems with integrating therapeutic DNA into the genome and the rapidly dividing nature of many cells prevent gene therapy from achieving any long-term benefits; (2) problems with the use of viral vectors, which are the carrier of choice in most gene therapy studies, including toxicity, virulence, immune and inflammatory responses, and issues with gene control and targeting; and (3) the chance of inducing a tumor by insertional mutagenesis. If the DNA is integrated in the wrong place in the genome, for example in a tumor suppressor gene, it could induce a tumor.
In addition to gene therapy, therapeutic strategies designed to combat HIV/AIDS have primarily relied upon small molecule drugs. Although some highly active antiretroviral therapy (HAART) treatments for HIV-1 have been therapeutically effective in the majority of patients, drug resistance and toxicity remain a concern with some individuals not responding to such therapy [1]. Alternative therapeutic strategies need to be developed to overcome these limitations.
Thus, it is of importance to find alternative approaches for delivery of therapeutic proteins or peptides in a combinatorial gene therapy setting.